1. Field of the Invention
The present invention relates to a photocurrent-to-voltage conversion apparatus including a clamping MOS transistor.
2. Description of the Related Art
Generally, in factor automation (FA) or the like, in order to electrically isolate an input from an output in a servo controller, a sequencer, an inverter or the like, a photocoupler has been developed. In such a photo coupler, a photocurrent-to-voltage conversion apparatus is used in a light receiving circuit thereof. Also, a photocurrent-to-voltage conversion apparatus is used in a light detection circuit in infrared communication or optical cable communication. Further, a photocurrent-to-voltage conversion apparatus is used in a light detection circuit for converting a laser reflection signal into an electrical digital signal of an optical disc unit.
A first prior art photocurrent-to-voltage conversion apparatus for a photodiode is constructed by an operational amplifier connected to the cathode of the photodiode and a negative feedback resistor connected between an output and an input of the operational amplifier (see: FIG. 4 of JP-A-61-41213). This will be explained later in detail.
In the above-described first prior art photocurrent-to-voltage conversion apparatus, however, when the strength of a light signal incident to the photodiode is very large, the operational amplifier would be saturated, so that waveform distortion between a photocurrent flowing through the photodiode and a detection voltage would be generated. Thus, an accurate light transmission would not be expected.
In a second prior art photocurrent-to-voltage conversion apparatus, a diode serving as a clamp element is connected in parallel with the negative feedback resistor of the first prior art photocurrent-to-voltage conversion apparatus (see: FIG. 5 of JP-A-61-41213). Also, in a third prior art photocurrent-to-voltage conversion apparatus, a diode-connected N-channel MOS transistor serving as a clamp element is connected in parallel with the negative feedback resistor of the first prior art photocurrent-to-voltage conversion apparatus (see: FIG. 1 of JP-A-61-41213). The second and third photocurrent-to-voltage conversion apparatuses also will be explained later in detail.
In the above-described second and third prior art photocurrent-to-voltage conversion apparatuses, however, when the difference in voltage between the input and output of the operational amplifier has a characteristic where the saturated difference is smaller than the clamp voltage of the clamp element, the operational amplifier is saturated before a current flows through the clamp element, so that the waveform distortion between the photocurrent and the detection voltage would still be generated.
Also, in the above-described second and third prior art photocurrent-to-voltage conversion apparatuses, when the difference in voltage between the input and output of the operational amplifier has a characteristic where the saturated difference is larger than the clamp voltage of the clamp element, a current flows through the clamp element before the operational amplifier is saturated, so that the detection voltage is clamped at a voltage lower than an expected value.